Network Working Group R. Bellis
Request for Comments: 5625 Nominet UK
BCP: 152 August 2009
Category: Best Current Practice
DNS Proxy Implementation Guidelines
Abstract
This document provides guidelines for the implementation of DNS
proxies, as found in broadband gateways and other similar network
devices.
Status of This Memo
This document specifies an Internet Best Current Practices for the
Internet Community, and requests discussion and suggestions for
improvements. Distribution of this memo is unlimited.
Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents in effect on the date of
publication of this document (http://trustee.ietf.org/license-info).
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document.
Bellis Best Current Practice [Page 1]

RFC 5625 DNS Proxy Implementation Guidelines August 20092. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. The Transparency Principle
It is not considered practical for a simple DNS proxy to implement
all current and future DNS features.
There are several reasons why this is the case:
o Broadband gateways usually have limited hardware resources.
o Firmware upgrade cycles are long, and many users do not routinely
apply upgrades when they become available.
o No one knows what those future DNS features will be or how they
might be implemented.
o Doing so would substantially complicate the configuration user
interface (UI) of the device.
Furthermore, some modern DNS protocol extensions (see, e.g., EDNS0
below) are intended to be used as "hop-by-hop" mechanisms. If the
DNS proxy is considered to be such a "hop" in the resolution chain,
then for it to function correctly, it would need to be fully
compliant with all such mechanisms.
[SAC035] shows that the more actively a proxy participates in the DNS
protocol, the more likely it is that it will somehow interfere with
the flow of messages between the DNS client and the upstream
recursive resolvers.
The role of the proxy should therefore be no more and no less than to
receive DNS requests from clients on the LAN side, forward those
verbatim to one of the known upstream recursive resolvers on the WAN
side, and ensure that the whole response is returned verbatim to the
original client.
It is RECOMMENDED that proxies should be as transparent as possible,
such that any "hop-by-hop" mechanisms or newly introduced protocol
extensions operate as if the proxy were not there.
Except when required to enforce an active security or network policy
(such as maintaining a pre-authentication "walled garden"), end-users
SHOULD be able to send their DNS queries to specified upstream
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RFC 5625 DNS Proxy Implementation Guidelines August 2009
resolvers, thereby bypassing the proxy altogether. In this case, the
gateway SHOULD NOT modify the DNS request or response packets in any
way.
4. Protocol Conformance4.1. Unexpected Flags and Data
The Transparency Principle above, when combined with Postel's
Robustness Principle [RFC0793], suggests that DNS proxies should not
arbitrarily reject or otherwise drop requests or responses based on
perceived non-compliance with standards.
For example, some proxies have been observed to drop any packet
containing either the "Authentic Data" (AD) or "Checking Disabled"
(CD) bits from DNSSEC [RFC4035]. This may be because [RFC1035]
originally specified that these unused "Z" flag bits "MUST" be zero.
However, these flag bits were always intended to be reserved for
future use, so refusing to proxy any packet containing these flags
(now that uses for those flags have indeed been defined) is not
appropriate.
Therefore, proxies MUST ignore any unknown DNS flags and proxy those
packets as usual.
4.2. Label Compression
Compression of labels as per Section 4.1.4 of [RFC1035] is optional.
Proxies MUST forward packets regardless of the presence or absence of
compressed labels therein.
4.3. Unknown Resource Record Types
[RFC3597] requires that resolvers MUST handle Resource Records (RRs)
of unknown type transparently.
All requests and responses MUST be proxied regardless of the values
of the QTYPE and QCLASS fields.
Similarly, all responses MUST be proxied regardless of the values of
the TYPE and CLASS fields of any Resource Record therein.
4.4. Packet Size Limits
[RFC1035] specifies that the maximum size of the DNS payload in a UDP
packet is 512 octets. Where the required portions of a response
would not fit inside that limit, the DNS server MUST set the
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RFC 5625 DNS Proxy Implementation Guidelines August 2009
"TrunCation" (TC) bit in the DNS response header to indicate that
truncation has occurred. There are however two standard mechanisms
(described in Sections 4.4.1 and 4.4.2) for transporting responses
larger than 512 octets.
Many proxies have been observed to truncate all responses at 512
octets, and others at a packet size related to the WAN MTU, in either
case doing so without correctly setting the TC bit.
Other proxies have been observed to remove the TC bit in server
responses that correctly had the TC bit set by the server.
If a DNS response is truncated but the TC bit is not set, then client
failures may result. In particular, a naive DNS client library might
suffer crashes due to reading beyond the end of the data actually
received.
Since UDP packets larger than 512 octets are now expected in normal
operation, proxies SHOULD NOT truncate UDP packets that exceed that
size. See Section 4.4.3 for recommendations for packet sizes
exceeding the WAN MTU.
If a proxy must unilaterally truncate a response, then the proxy MUST
set the TC bit. Similarly, proxies MUST NOT remove the TC bit from
responses.
4.4.1. TCP Transport
Should a UDP query fail because of truncation, the standard fail-over
mechanism is to retry the query using TCP, as described in Section6.1.3.2 of [RFC1123].
Whilst TCP transport is not strictly mandatory, it is supported by
the vast majority of stub resolvers and recursive servers. Lack of
support in the proxy prevents this fail-over mechanism from working.
DNS proxies MUST therefore be prepared to receive and forward queries
over TCP.
Note that it is unlikely that a client would send a request over TCP
unless it had already received a truncated UDP response. Some
"smart" proxies have been observed to first forward any request
received over TCP to an upstream resolver over UDP, only for the
response to be truncated, causing the proxy to retry over TCP. Such
behaviour increases network traffic and causes delay in DNS
resolution since the initial UDP request is doomed to fail.
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RFC 5625 DNS Proxy Implementation Guidelines August 2009
Therefore, whenever a proxy receives a request over TCP, the proxy
SHOULD forward the query over TCP and SHOULD NOT attempt the same
query over UDP first.
4.4.2. Extension Mechanisms for DNS (EDNS0)
The "Extension Mechanism for DNS" [RFC2671] was introduced to allow
the transport of larger DNS packets over UDP and also to allow for
additional request and response flags.
A client may send an OPT Resource Record (OPT RR) in the Additional
Section of a request to indicate that it supports a specific receive
buffer size. The OPT RR also includes the "DNSSEC OK" (DO) flag used
by DNSSEC to indicate that DNSSEC-related RRs should be returned to
the client.
However, some proxies have been observed to either reject (with a
FORMERR response code) or black-hole any packet containing an OPT RR.
As per Section 4.1, proxies MUST NOT refuse to proxy such packets.
4.4.3. IP Fragmentation
Support for UDP packet sizes exceeding the WAN MTU depends on the
gateway's algorithm for handling fragmented IP packets. Several
methods are possible:
1. Fragments are dropped.
2. Fragments are forwarded individually as they're received.
3. Complete packets are reassembled on the gateway and then re-
fragmented (if necessary) as they're forwarded to the client.
Method 1 above will cause compatibility problems with EDNS0 unless
the DNS client is configured to advertise an EDNS0 buffer size
limited to the WAN MTU less the size of the IP header. Note that RFC2671 does recommend that the path MTU should be taken into account
when using EDNS0.
Also, whilst the EDNS0 specification allows for a buffer size of up
to 65535 octets, most common DNS server implementations do not
support a buffer size above 4096 octets.
Therefore (irrespective of which of the above methods is in use),
proxies SHOULD be capable of forwarding UDP packets up to a payload
size of at least 4096 octets.
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RFC 5625 DNS Proxy Implementation Guidelines August 2009
NB: in theory, IP fragmentation may also occur if the LAN MTU is
smaller than the WAN MTU, although the author has not observed such a
configuration in use on any residential broadband service.
4.5. Secret Key Transaction Authentication for DNS (TSIG)
[RFC2845] defines TSIG, which is a mechanism for authenticating DNS
requests and responses at the packet level.
Any modifications made to the DNS portions of a TSIG-signed query or
response packet (with the exception of the Query ID) will cause a
TSIG authentication failure.
DNS proxies MUST implement Section 4.7 of [RFC2845] and either
forward packets unchanged (as recommended above) or fully implement
TSIG.
As per Section 4.3, DNS proxies MUST be capable of proxying packets
containing TKEY [RFC2930] Resource Records.
NB: any DNS proxy (such as those commonly found in WiFi hotspot
"walled gardens") that transparently intercepts all DNS queries and
that returns unsigned responses to signed queries, will also cause
TSIG authentication failures.
5. DHCP's Interaction with DNS
Whilst this document is primarily about DNS proxies, most consumers
rely on DHCP [RFC2131] to obtain network configuration settings.
Such settings include the client machine's IP address, subnet mask,
and default gateway, but also include DNS-related settings.
It is therefore appropriate to examine how DHCP affects client DNS
configuration.
5.1. Domain Name Server (DHCP Option 6)
Most gateways default to supplying their own IP address in the DHCP
"Domain Name Server" option [RFC2132]. The net result is that
without explicit re-configuration many DNS clients will, by default,
send queries to the gateway's DNS proxy. This is understandable
behaviour given that the correct upstream settings are not usually
known at boot time.
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RFC 5625 DNS Proxy Implementation Guidelines August 2009
Most gateways learn their own DNS settings via values supplied by an
ISP via DHCP or PPP over the WAN interface. However, whilst many
gateways do allow the device administrator to override those values,
some gateways only use those supplied values to affect the proxy's
own forwarding function, and do not offer these values via DHCP.
When using such a device, the only way to avoid using the DNS proxy
is to hard-code the required values in the client operating system.
This may be acceptable for a desktop system but it is inappropriate
for mobile devices that are regularly used on many different
networks.
As per Section 3, end-users SHOULD be able to send their DNS queries
directly to specified upstream resolvers, ideally without hard-coding
those settings in their stub resolver.
It is therefore RECOMMENDED that gateways SHOULD support device-
administrator configuration of values for the "Domain Name Server"
DHCP option.
5.2. Domain Name (DHCP Option 15)
A significant amount of traffic to the DNS Root Name Servers is for
invalid top-level domain names, and some of that traffic can be
attributed to particular equipment vendors whose firmware defaults
this DHCP option to specific values.
Since no standard exists for a "local" scoped domain name suffix, it
is RECOMMENDED that the default value for this option SHOULD be
empty, and that this option MUST NOT be sent to clients when no value
is configured.
5.3. DHCP Leases
It is noted that some DHCP servers in broadband gateways offer, by
default, their own IP address for the "Domain Name Server" option (as
described above) but then automatically start offering the upstream
servers' addresses once they've been learnt over the WAN interface.
In general, this behaviour is highly desirable, but the effect for
the end-user is that the settings used depend on whether the DHCP
lease was obtained before or after the WAN link was established.
If the DHCP lease is obtained whilst the WAN link is down, then the
DHCP client (and hence the DNS client) will not receive the correct
values until the DHCP lease is renewed.
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RFC 5625 DNS Proxy Implementation Guidelines August 2009
Whilst no specific recommendations are given here, vendors may wish
to give consideration to the length of DHCP leases and to whether
some mechanism for forcing a DHCP lease renewal might be appropriate.
Another possibility is that the learnt upstream values might be
persisted in non-volatile memory such that on reboot the same values
can be automatically offered via DHCP. However, this does run the
risk that incorrect values are initially offered if the device is
moved or connected to another ISP.
Alternatively, the DHCP server might only issue very short (i.e., 60
second) leases while the WAN link is down, only reverting to more
typical lease lengths once the WAN link is up and the upstream DNS
servers are known. Indeed, with such a configuration it may be
possible to avoid the need to implement a DNS proxy function in the
broadband gateway at all.
6. Security Considerations
This document introduces no new protocols. However, there are some
security-related recommendations for vendors that are listed here.
6.1. Forgery Resilience
Whilst DNS proxies are not usually full-feature resolvers, they
nevertheless share some characteristics with them.
Notwithstanding the recommendations above about transparency, many
DNS proxies are observed to pick a new Query ID for outbound requests
to ensure that responses are directed to the correct client.
NB: changing the Query ID is acceptable and compatible with proxying
TSIG-signed packets since the TSIG signature calculation is based on
the original message ID, which is carried in the TSIG RR.
It has been standard guidance for many years that each DNS query
should use a randomly generated Query ID. However, many proxies have
been observed picking sequential Query IDs for successive requests.
It is strongly RECOMMENDED that DNS proxies follow the relevant
recommendations in [RFC5452], particularly those in Section 9.2
relating to randomisation of Query IDs and source ports. This also
applies to source port selection within any NAT function.
If a DNS proxy is running on a broadband gateway with NAT that is
compliant with [RFC4787], then it SHOULD also follow the
recommendations in Section 10 of [RFC5452] concerning how long DNS
state is kept.
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RFC 5625 DNS Proxy Implementation Guidelines August 20096.2. Interface Binding
Some gateways have been observed to have their DNS proxy listening on
both internal (LAN) and external (WAN) interfaces. In this
configuration, it is possible for the proxy to be used to mount
reflector attacks as described in [RFC5358].
The DNS proxy in a gateway SHOULD NOT, by default, be accessible from
the WAN interfaces of the device.
6.3. Packet Filtering
The Transparency and Robustness Principles are not entirely
compatible with the deep packet-inspection features of security
appliances such as firewalls, which are intended to protect systems
on the inside of a network from rogue traffic.
However, a clear distinction may be made between traffic that is
intrinsically malformed and that which merely contains unexpected
data.
Examples of malformed packets that MAY be dropped include:
o invalid compression pointers (i.e., those that point outside of
the current packet or that might cause a parsing loop)
o incorrect counts for the Question, Answer, Authority, and
Additional Sections (although care should be taken where
truncation is a possibility)
Dropped packets will cause the client to repeatedly retransmit the
original request, with the client only detecting the error after
several retransmit intervals.
In these circumstances, proxies SHOULD synthesise a suitable DNS
error response to the client (i.e., SERVFAIL) instead of dropping the
packet completely. This will allow the client to detect the error
immediately.
7. Acknowledgements
The author would particularly like to acknowledge the assistance of
Lisa Phifer of Core Competence. In addition, the author is grateful
for the feedback from the members of the DNSEXT Working Group.
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